The present invention relates to paper machines, board machines, pulp machines or equivalent. More specifically, the present invention relates to a headbox [according to the preamble of claim 1, which headbox is ] suitable for high basis weights and high fibre suspension consistencies.
This kind of conventional headbox of a paper machine or board machine or pulp machine or equivalent is thus, for example, of the kind comprising a turbulence part for producing a turbulent fibre suspension flow, into which turbulence part a fibre suspension is coming from a distributor means of the fibre suspension, advantageously a tube bank, and a slice part after the turbulence part, from which slice part the fibre suspension flows to a forming section of a fibrous web.
U.S. Pat. No. 4,285,767, Beloit Corp., discloses a headbox for a high consistency fibre suspension. The U.S. patent proposes a flow passage, which is adjustable in its cross-sectional flow area and narrows stepwise towards a slice part, for a fibre suspension flowing in a foamed state in order that it might be assured that the fibres are uniformly distributed in the fibre suspension. The fibre suspension is passed into the stepped passage from a distributor means of the fibre suspension through tube passages; according to the U.S. patent, the cross-sectional flow area is adjusted by moving a spindle part of the headbox axially away from and towards a slice part in order to increase and, correspondingly, to decrease the cross-sectional flow area.
The use of high consistency and the suitability of today's headboxes for use as high consistency headboxes involve a risk of cavitation and the fact that the use of high consistency requires fluidization of the fibre suspension in order that the fibres might be caused to be distributed uniformly in the fibre suspension and that the quality of the web might be made satisfactory. On the other hand, increase of fluidization limits increase of basis weight, for example, in pulp-drying machines and board machines.